io_uring: clean __io_import_iovec()

Apparently, implicit 0 to NULL conversion with ERR_PTR is not
recommended and makes some tooling like Smatch to complain. Handle it
explicitly, compilers are perfectly capable to optimise it out.

Link: https://lore.kernel.org/all/20211108134937.GA2863@kili/Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/5c6ed369ad95075dab345df679f8677b8fe66656.1637524285.git.asml.silence@gmail.comSigned-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -3179,10 +3179,12 @@ static struct iovec *__io_import_iovec(int rw, struct io_kiocb *req,
 	size_t sqe_len;
 	ssize_t ret;
 
-	BUILD_BUG_ON(ERR_PTR(0) != NULL);
-
-	if (opcode == IORING_OP_READ_FIXED || opcode == IORING_OP_WRITE_FIXED)
-		return ERR_PTR(io_import_fixed(req, rw, iter));
+	if (opcode == IORING_OP_READ_FIXED || opcode == IORING_OP_WRITE_FIXED) {
+		ret = io_import_fixed(req, rw, iter);
+		if (ret)
+			return ERR_PTR(ret);
+		return NULL;
+	}
 
 	/* buffer index only valid with fixed read/write, or buffer select  */
 	if (unlikely(req->buf_index && !(req->flags & REQ_F_BUFFER_SELECT)))
@@ -3200,15 +3202,18 @@ static struct iovec *__io_import_iovec(int rw, struct io_kiocb *req,
 		}
 
 		ret = import_single_range(rw, buf, sqe_len, s->fast_iov, iter);
-		return ERR_PTR(ret);
+		if (ret)
+			return ERR_PTR(ret);
+		return NULL;
 	}
 
 	iovec = s->fast_iov;
 	if (req->flags & REQ_F_BUFFER_SELECT) {
 		ret = io_iov_buffer_select(req, iovec, issue_flags);
-		if (!ret)
-			iov_iter_init(iter, rw, iovec, 1, iovec->iov_len);
-		return ERR_PTR(ret);
+		if (ret)
+			return ERR_PTR(ret);
+		iov_iter_init(iter, rw, iovec, 1, iovec->iov_len);
+		return NULL;
 	}
 
 	ret = __import_iovec(rw, buf, sqe_len, UIO_FASTIOV, &iovec, iter,